A typical galaxy spectrum at different redshifts
This animation shows how the typical spectrum of a galaxy varies with the redshift of the galaxy, as a consequence of the expansion of the Universe. The bulk of the observed radiation peaks at wavelengths well below 1 millimetre for relatively nearby galaxies (z~0–1) whereas, as the galaxy redshift increases (z~3–6), the peak of the observed radiation moves towards longer and longer wavelengths, approaching 1 millimetre.
This illustrates why astrophysicists believe that the fluctuations of the Cosmic Infrared Background (CIB) observed at a lower frequency (corresponding to a longer wavelength) might be generated by galaxies that are further away than those giving rise to CIB fluctuations observed at higher frequencies (corresponding to shorter wavelengths).
The four straight lines represent four of the frequencies probed by the High Frequency Instrument (HFI) on board Planck: from left to right, 857 GHz, 545 GHz, 353 GHz and 217 GHz, respectively.
Another effect depicted in the animation is the cosmological dimming, or the fact that galaxies at higher redshifts, being further away, appear fainter to us.